1. Field of the Invention
The invention relates to an extrusion die for the production of flat webs of sizable thickness made of thermoplastic; they are also designated as flat-sheet dies.
2. Discussion of Background
For extrusion of thin sheets, flat-sheet dies without a restrictor bar but with elastic die outlet lips, so-called "flex lips," are used. By their elasticity, they allow a change in thickness of the flow channel of the thermoplastic melt from the manifold, up to the die lips, by up to about one half millimeter. For this purpose, multiple adjusting screws or thermal expansion pins, which act on the elastic lip, are used. When the thickness of the extruded sheet web is under 2 mm, an adjusting lift of one half millimeter is sufficient to regulate the flow of melt to the die lips and to compensate for possible irregularities in the transverse direction.
According to German utility model G 88 13 801, the positioning force is allowed to act on the elastic lip by multiple movably placed plates. This enables a more precise adjustment, for example for bead size adjustment on a downstream polishing roller, but the adjusting lift is on an order of magnitude which is matched to the extrusion of thin sheet webs.
For extrusion of thermoplastics into sheet webs with a thickness of more than 2 mm, flat-sheet dies with a restrictor bar are generally used. This is understood to be a bar projecting between the manifold and the die lips in the restricting passage of the die, which is placed movably in a groove and can be sunk by a suitable adjusting mechanism to different depths in the melt stream. In this way, melt streams of considerable thickness can also be controlled.
A number of drawbacks are connected with the use of a restrictor bar. It has to be produced separately and fitted tightly into the groove provided for it. The penetration of the plastic melt in the gap between the groove wall and the restrictor bar has to be prevented by sealing strips, which is only conditionally possible because of the material requirements on the sealing means which are difficult to meet. For design reasons, the restrictor bar has to be at a considerable distance from the die lips, where the melt is still under a very high pressure. Therefore it has to be very rigidly designed, so that it is only conditionally suitable for a fine control over the die width. The required positioning forces are considerable. The great pressure drop of the melt from the area of the restrictor bar up to the die lips can cause crosscurrents, by which the compensating action of the restrictor bar is partially destroyed again. Further, so-called dead points in the melt channel at the beginning and at the end of the restrictor bar are difficult to avoid. As a result, portions of the melt can be stopped and only be moved after the beginning of a thermal decomposition, so that streaks of decomposed material occur in the produced web.